The report notes that these top eight firms spent $540 million on lobbying from 2007 to 2009. They filed 332 lobbying reports that mentioned taxes and named 491 different tax bills in those reports.

The top eight companies that spent the most on lobbying were Exxon Mobil, Verizon Communications, General Electric, AT&T, Altria, Amgen, Northrop Grumman and Boeing. Exxon Mobil spent the most, some $81.92 million from 2007 to 2009.

AT&T recorded the largest tax reduction, with its tax rate falling from 34.0 percent to negative 6.4 percent from 2007 to 2010, or an estimated reduction of more than $7.3 billion. Altria, the parent company of Philip Morris, had the smallest decline from 2007 to 2010, with its rate declining from 28.9 percent to 27.4 percent. Six of the top eight companies saw declines of at least 7 percentage points.

The report comes as both President Barack Obama and Republican Party's presumptive nominee, Mitt Romney, have proposed lowering corporate tax rates. Obama has proposed lowering the corporate tax rate from 35 percent to 28 percent but eliminating loopholes and deductions. American manufacturers would get a bigger tax cut, having an effective rate of no more than 25 percent.

Despite the fact that the corporate tax rate is the highest in the developed world, few companies pay the full amount. A November 2011 study by the Citizens for Tax Justice and the Institute on Taxation and Economic policy found that the largest and most profitable 280 corporations paid on average an effective rate of 18.5 percent. Accelerated depreciation, use of stock options for compensation, industry-specific tax breaks, offshore tax sheltering and a weakened corporate alternative minimum tax were among the causes for the lower effective rates, according to that report.

To avoid arriving at tax rates driven by one-time charges, write-offs or expenses, the Sunlight Foundation took the top 200 companies that had reported tax rates that ranged from negative 50 percent to 50 percent in 2007 and 2010 and with a positive income in 2007.

The report noted that while it's impossible to tell exactly why these companies' rates fell, the likelihood of six of the top eight companies lowering their rates "by at least seven percentage points purely by random chance is less than 1 in 100,000."